Soil-structure interaction under multi-directional earthquake loading

Abstract

The dynamic interaction between the soil and the structure resting on it during

earthquakes can alter the response characteristics both of the structure and the soil.

Despite significant efforts over the past decades, the interaction effect is not yet

fully understood and is sometimes misunderstood. In the context of performance

based design, there remain a number of uncertainties to be addressed seriously.

Current practice of seismic soil-structure response analysis has tended to focus on

the effect of horizontal motion although actual ground motions are comprised of

both horizontal and vertical components. In several recent earthquakes, very

strong vertical ground motions have been recorded, raising great concern over the

potential effect of vertical motion on engineering structures. To address this

emerging problem, seismic response considering the soil-structure interaction

effect to both vertical and horizontal earthquake motions needs to be investigated.

This thesis presents a simple and practical framework for the analysis of site

response and soil-structure interaction to both horizontal and vertical earthquake

motions, which can take into account the soil nonlinearity and material damping

effect. The analysis procedure involves the use of the dynamic stiffness matrix

method and equivalent-linear approach and is built in the modern MATLAB

environment to take the full advantages of the matrix operations in MATLAB.

The input motions can be specified at the soil–bedrock interface or a rock

outcropping. A detailed assessment of the procedure is provided to illustrate that

the procedure is able to produce acceptable predictions of both vertical and

horizontal response of soil-structure systems. It is shown that soil nonlinearity

plays an important role in altering the response of the structure and soil, and the

methods of analysis for soil-structure interaction adopted in current engineering

practice may not be able to adequately account for soil nonlinearity. Furthermore,

effects of a number of influencing factors, such as material damping ratio,

Poisson’s ratio of soil, intensity and location of input motion and the embedment

ratio of the foundation are examined, leading to several useful implications for

seismic engineering practice.